Class i and siblings - Page 13 - diyAudio
Go Back   Home > Forums > Amplifiers > Solid State

Solid State Talk all about solid state amplification.

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 5th January 2012, 06:07 PM   #121
diyAudio Member
 
Edmond Stuart's Avatar
 
Join Date: Nov 2003
Location: Amsterdam
Needed.
At room temp. the voltage drop across these resistors should be about 7mV (=33-26mV)
Cheers,
E.
__________________
Een volk dat voor tirannen zwicht, zal meer dan lijf en
goed verliezen dan dooft het licht…(H.M. van Randwijk)

Last edited by Edmond Stuart; 5th January 2012 at 06:13 PM.
  Reply With Quote
Old 5th January 2012, 06:56 PM   #122
Waly is offline Waly  United Kingdom
diyAudio Member
 
Waly's Avatar
 
Join Date: Jan 2011
Location: London
This triple configuration does not appear to be stable, in any non-trivial (>5nF) capacitive load. See attachment, load is 10nF/step. For output swings toward the rails, I would expect the stability margins to be even worse. This is for zero source impedance, which is not necessary realistic - it is almost impossible to fully analyze the stability of this triple configuration in isolation from the preceding transimpedance gain stage.

Unfortunately, a Zobel cell, output inductance, driver BC cap are here not helping.
Attached Images
File Type: jpg stability.jpg (172.7 KB, 244 views)

Last edited by Waly; 5th January 2012 at 07:00 PM.
  Reply With Quote
Old 5th January 2012, 07:30 PM   #123
Waly is offline Waly  United Kingdom
diyAudio Member
 
Waly's Avatar
 
Join Date: Jan 2011
Location: London
Default Output

BTW, under the same conditions, this is the output for a 25V/20KHz input signal and 50nF capacitive load.
Attached Images
File Type: jpg output.jpg (133.1 KB, 236 views)
  Reply With Quote
Old 5th January 2012, 09:33 PM   #124
miralin is offline miralin  Russian Federation
diyAudio Member
 
Join Date: Dec 2010
Default Fodity

Hi Edmond
Dr. Bernard M. Oliver (Barney) did not mention any optimum point in his article (http://www.hpl.hp.com/hpjournal/pdfs...Fs/1971-02.pdf). He proposes as optimal the whole interval from 13 to 26 millivolts across one resistor (p.13), so any figure is right.
But only for the primitive case, in which - as necessary for organizing Class AB nonlinearities - are utilized nonlinearities of the output devices. These nonlinearities are very inconvenient (susceptible to thermal shocks, sluggish, asymmetric etc.). And they are not fod, i.e. free of (crossover) distortion.
So some people like you and Kendall eliminate them by high amount of NFB thru powerful drivers.
Then you should create (synthesize, tailor) your own nonlinearities instead, preferably fod. Only this property (let’s call it fodity) must be thermally stable, precise, independent of frequency etc.
So Oliver’s analysis and Oliver’s criteria are not applicable. You’ve already noted this in #28.
And if, for example, basics of Kendall’s nonlinearities dictate that (like in most cases with bjt’s) Iq must grow with temperature, one cannot try to stabilize it.
And what about fodity? This is simply a “main condition” from #113: difference between the two nonlinear functions Ip(Vin) and In(Vin) must result in a linear function Gm*Vin.
For your simulation in #115 you define Vin as the mean voltage of the top and bottom driver input.
But it's no use in this case, like “Barny Oliver with all his wisdom”, because no one is interesting about what happens deep inside the circuitry. Seems that more useful would be
Gm=del(I(R37))/del(V(in)-V(out)), where (in) is the left node of R1 or R3.
For Class-I theoretically calculated Gm is exactly equal to 1/R24 (#3 in this thread), you can verify it by simulation. And what about ABII?
Cheers Mir
  Reply With Quote
Old 6th January 2012, 10:12 AM   #125
dadod is offline dadod  Croatia
diyAudio Member
 
dadod's Avatar
 
Join Date: Apr 2006
Location: Zagreb
Quote:
Originally Posted by Waly View Post
BTW, under the same conditions, this is the output for a 25V/20KHz input signal and 50nF capacitive load.
Here is 30V/20kHz input and 100nF capacitive load, no problem.
Damir
Attached Images
File Type: jpg EDMOND-OPS-1b-100n.jpg (153.7 KB, 238 views)
  Reply With Quote
Old 6th January 2012, 01:12 PM   #126
diyAudio Member
 
Edmond Stuart's Avatar
 
Join Date: Nov 2003
Location: Amsterdam
Default VT & Gm

Quote:
Originally Posted by miralin View Post
Hi Edmond
Dr. Bernard M. Oliver (Barney) did not mention any optimum point in his article (http://www.hpl.hp.com/hpjournal/pdfs...Fs/1971-02.pdf). He proposes as optimal the whole interval from 13 to 26 millivolts across one resistor (p.13), so any figure is right.
Hi Mir,

Perhaps I'm wrong, but I thought there's general agreement that Vq = VT is optimal. Although Oliver didn't say this explicitly, his fig.6 (see 1st pic. below) suggest that this is indeed the case (go*R=1).
On the other hand, the result of some of my simulations reveals that the optimal Vq depends on the signal level. I found optima between 14 and 38mV (@ room temp.)
Furthermore, I also got some contradicting result: On page 28 I mentioned some increase of the distortion when Vq=VT, while on another occasion I found a little improvement, though this was with a slightly different auto-bias circuit.
So, I think there's no need to strictly adhere to the Vq=VT rule (as you said already).

Quote:
But only for the primitive case, in which - as necessary for organizing Class AB nonlinearities - are utilized nonlinearities of the output devices. These nonlinearities are very inconvenient (susceptible to thermal shocks, sluggish, asymmetric etc.). And they are not fod, i.e. free of (crossover) distortion.
So some people like you and Kendall eliminate them by high amount of NFB thru powerful drivers.
Then you should create (synthesize, tailor) your own nonlinearities instead, preferably fod. Only this property (let’s call it fodity) must be thermally stable, precise, independent of frequency etc.
So Oliver’s analysis and Oliver’s criteria are not applicable. You’ve already noted this in #28.
And if, for example, basics of Kendall’s nonlinearities dictate that (like in most cases with bjt’s) Iq must grow with temperature, one cannot try to stabilize it.
And what about fodity? This is simply a “main condition” from #113: difference between the two nonlinear functions Ip(Vin) and In(Vin) must result in a linear function Gm*Vin.
For your simulation in #115 you define Vin as the mean voltage of the top and bottom driver input.
But it's no use in this case, like “Barny Oliver with all his wisdom”, because no one is interesting about what happens deep inside the circuitry.
IMHO, it's most relevant to look at the transfer function relative to this 'deep inside input'. The point is that any nonlinearity between this input and the output directly translate to the overall performance. Besides, as the phase compensation stuff is connected to these driver inputs, the overall performance at higher frequencies is even more determined by this transfer function.

Quote:
Seems that more useful would be Gm=del(I(R37))/del(V(in)-V(out)), where (in) is the left node of R1 or R3.
Agreed, that's also very useful, in particular when comparing Class-I with AB2.

Quote:
For Class-I theoretically calculated Gm is exactly equal to 1/R24 (#3 in this thread), you can verify it by simulation. And what about ABII?
Cheers Mir
Well, more precisely, for positive signals it's indeed equal to 1/R24. However.......... for negative signals it's equal to 1/R25 and you know of course what that means. Perhaps, this is the weakest point of Class-I, at least to me. I would never build this OPS.

Regarding AB2, in post 115, I made again a stupid mistake. I wrote: "So the incremental transconductance becomes: gm = del( I(R37) / del (0.5*( V(B1) + V(B2) ) - V(out) )"
That's okay. Then I wrote: "To my own surprise, gm is almost perfectly constant. At Vout =0 there is a tiny dip of about 2.7%. No gm doubling at all."
That's BS. No wonder I was surprised. Have a closer look at the expression for gm: Del(I(R37))/Del(0.5*(V(b1)+V(b2))-V(out)) . Do you see the error?

Fig.2, green curve, shows the correct 'internal' gm of the amp on page 41. It's far from constant; it varies from 3 to 4.6 Siemens. I also tried to plot the overall gm, but at some output levels, it's becomes + or - infinity (weird!). So I've plot the inverse gm, i.e. the incremental Zo, which varies from -0.13mOhm to +1mOhm. See the blue curve. The reason for this weird behavior seems to be the cross-connection of R23 and R24. If I replace them by CCSes, Zo varies from +0.3mOhm to +0.9mOhm, in about the same manner as in fig.3.

The 3rd pic. shows the 'internal' (green) and overall gm (blue) of AB2 with a MOSFET OPS. This one doesn't have the resistor cross-connection in the driver stage. So gm behaves 'normal'. Purple is the incremental Zo.

BTW, what do you mean by 'fod'? 'Free of distortion', right?

Cheers,
E.
Attached Images
File Type: jpg Oliver-Xover.jpg (47.1 KB, 221 views)
File Type: png AB2-BJT-PP2-GM.png (19.1 KB, 208 views)
File Type: png AB2-MOS-GM.png (21.7 KB, 64 views)
__________________
Een volk dat voor tirannen zwicht, zal meer dan lijf en
goed verliezen dan dooft het licht…(H.M. van Randwijk)

Last edited by Edmond Stuart; 6th January 2012 at 01:16 PM.
  Reply With Quote
Old 6th January 2012, 01:33 PM   #127
diyAudio Member
 
Edmond Stuart's Avatar
 
Join Date: Nov 2003
Location: Amsterdam
Quote:
Originally Posted by dadod View Post
Here is 30V/20kHz input and 100nF capacitive load, no problem.
Damir
Hi Damir,

What if you connect that cap directly to the output, that is, before the Zobel network? Also, what happens if you add parasitic inductances of the leads and traces, in particular to the OPS trannies?

Cheers,
E.
__________________
Een volk dat voor tirannen zwicht, zal meer dan lijf en
goed verliezen dan dooft het licht…(H.M. van Randwijk)
  Reply With Quote
Old 6th January 2012, 02:47 PM   #128
dadod is offline dadod  Croatia
diyAudio Member
 
dadod's Avatar
 
Join Date: Apr 2006
Location: Zagreb
Quote:
Originally Posted by Edmond Stuart View Post
Hi Damir,

What if you connect that cap directly to the output, that is, before the Zobel network? Also, what happens if you add parasitic inductances of the leads and traces, in particular to the OPS trannies?

Cheers,
E.
Hi Edmond,
I don't know how much capacitance is realistic to have before Zobel network but with 100nF it is not stable, with 10nF it is. Next test is with parasitic inductances(10uH I think that is realistic) and 1nF before Zobel and 2uF after and no problem there.
Damir
Attached Images
File Type: jpg EDMOND-OPS-1b-100n-b.jpg (151.5 KB, 86 views)
File Type: jpg EDMOND-OPS-1b-100n-c.jpg (156.4 KB, 81 views)
  Reply With Quote
Old 6th January 2012, 03:11 PM   #129
diyAudio Member
 
Edmond Stuart's Avatar
 
Join Date: Nov 2003
Location: Amsterdam
Quote:
Originally Posted by dadod View Post
Hi Edmond,
I don't know how much capacitance is realistic to have before Zobel network but with 100nF it is not stable, with 10nF it is.
Hi Damir,

Most amps don't like 100nF (even Bob's HEC amp). OTOH, 10nF is a bit low.

Quote:
Next test is with parasitic inductances(10uH I think that is realistic) and 1nF before Zobel and 2uF after and no problem there.
Damir
10uH?No! 10 to 50nH in series with the legs of the OP devices (~10nH/cm).

Cheers,
E.
__________________
Een volk dat voor tirannen zwicht, zal meer dan lijf en
goed verliezen dan dooft het licht…(H.M. van Randwijk)
  Reply With Quote
Old 6th January 2012, 04:00 PM   #130
dadod is offline dadod  Croatia
diyAudio Member
 
dadod's Avatar
 
Join Date: Apr 2006
Location: Zagreb
Quote:
Originally Posted by Edmond Stuart View Post
Hi Damir,

Most amps don't like 100nF (even Bob's HEC amp). OTOH, 10nF is a bit low.
Hi Edmond,
Before Zobel 10nF is to much I think.

Quote:
Originally Posted by Edmond Stuart View Post
10uH?No! 10 to 50nH in series with the legs of the OP devices (~10nH/cm).

Cheers,
E.
Sorry if you look at schematic you will see that I used 10nH not 10uH as I wrote.

Damir
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Class-AB meets Class-D: Yamaha's EEEngine Topology - where are this Diy Projects? tiefbassuebertr Solid State 29 14th April 2014 04:12 PM
Collection of Class B topologies <100mA Idle and Sound closest by Class A tiefbassuebertr Solid State 37 27th July 2012 08:04 AM
Can a Class AB PP amp be said to be operating in Class A at low signal levels? ray_moth Tubes / Valves 19 23rd January 2009 07:52 PM
How about a round-up of Class A kit power amps, or collectable vintage class A? Brisso57 Solid State 4 14th February 2007 10:30 AM


New To Site? Need Help?

All times are GMT. The time now is 02:32 PM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright ©1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2